With reference to FIG. 4, a conventional electron-beam apparatus comprises an electron gun (not shown in FIG. 4) that emits an electron beam EB along an axis AX. The electron beam EB forms a crossover CO1 and is then focused by a condenser lens 50 and deflected by a deflector 51. The electron beam then irradiates a mask 1 that defines a pattern to be transferred to a sensitized substrate 5. The sensitized substrate 5 is generally a semiconductor wafer or a glass plate that is coated with an electron-beam-sensitive resist.
The electron beam EB is selectively deflected by the deflector 51 to exemplary deflected electron beams EB.sub.1, EB.sub.2, EB.sub.3. The beams EB.sub.1, EB.sub.2, EB.sub.3 irradiate portions of an exemplary subfield 1a on the mask 1. The beam EB.sub.1 corresponds to a deflection of the electron beam EB such that the center of the subfield 1a is irradiated. The deflected beams EB.sub.2, EB.sub.3 irradiate respective perimeter portions of the subfield 1a. The electron beam EB is shaped so that rectangular portions of the subfield 1a are irradiated by the respective beams EB.sub.1, EB.sub.2, EB.sub.3.
After transmission by the mask 1, the electron beam EB is directed to the sensitized substrate 5 by a first projection lens 3 and a second projection lens 4. The first projection lens 3 consists of deflectors 31, 32, and focus correctors 35, 36 and the second projection lens 4 consists of deflectors 33, 34, and focus correctors 37, 38. The lenses 3, 4 image the subfield 1a onto the sensitized substrate 5.
The mask 1 and the sensitized substrate 5 are retained by a mask stage and a wafer stage, respectively (not shown in FIG. 4). The mask and wafer stages move continuously in opposite directions while the electron beam EB is deflected by the deflector 51 perpendicular to and along the direction of motion of the stages. The sensitized substrate 5 is exposed to the patterns from the entire mask 1 by sequentially exposing the mask to the pattern in each of a plurality of subfields similar to the subfield 1a. If the subfields are large, then a large area of the sensitized substrate 5 can be irradiated by at once and the time required to expose the sensitized substrate 5 can be reduced.
As shown in FIG. 4, the deflector 41 deflects the electron beam EB toward the axis AX to an aperture 6 placed at a crossover CO2. The electron beam EB is then deflected by the deflector 42 so that the electron beam EB propagates parallel to the axis AX. The projection lenses 3, 4 form a so-called variable-axis lens (VAL), and the deflectors 31-34 are VAL deflectors and the focus correctors 35-38 are VAL focus correctors. If B.sub.o (z) is the axial component of the magnetic field on the axis AX produced by the projection lenses 3, 4 and a is the distance from the axis AX to the center of the subfield 1a, then the magnetic field B.sub.rVAL produced by the VAL deflectors 31-34 is given by Equation 1: ##EQU1## where K.sub.1 is a constant. The VAL focus correctors 35-38 produce a magnetic field B.sub.zVAL given by Equation 2: ##EQU2## where K.sub.2 is a constant. The magnetic fields B.sub.rVAL and B.sub.zVAl shift the effective optical axis away from and parallel to the axis AX. As a result, the electron beam EB propagates along the effective axis of the projection lenses 3, 4, reducing off-axis aberrations.
Conventional charged-particle beam exposure apparatus have several significant limitations. If the deflection of the electron beam EB by the deflector 51 is large, then the deflection of the electron beam EB by the deflectors 41, 42 is also large. These large deflections introduce aberrations into the image of the subfield 1a. Because the deflection by the deflector 51 must be kept small to preserve image resolution, the size of the subfield 1a is correspondingly small and throughput is limited.